1. Field of the Invention
The present invention relates to a method for assessing and guaranteeing the thermal hygiene efficiency for items to be washed in a dishwasher during the cleaning process.
2. Description of the Background Art
For cleaning dishes in the commercial sector, multi-tank dishwashers are nowadays used in addition to single-chamber dishwashers. In these multi-tank dishwashers, the items to be cleaned are transported by a conveyor device through the various zones of the dishwasher. Multi-tank dishwashers generally comprise at least one washing zone, at least one final-rinse zone and, optionally, a drying zone. Multi-tank dishwashers, in which items to be cleaned pass through various treatment zones, are generally designed as conveyor-belt machines or basket transport machines. A common feature of both designs is that the items to be cleaned are transported continuously by the transporting means through the individual treatment zones. The individual treatment zones are usually designed as chambers that have openings in the transport direction of the conveyor device, through which openings the items to be cleaned are transported by means of the conveyor device.
At a start of operation of a multi-tank dishwasher, the washing water tank in the washing zone is filled with fresh water and heated to the preset washing tank temperature. Detergent is also added to the washing water. This applies analogously in cases where several washing zones are arranged in succession. The washing zone normally has a pump for circulating the washing water, which pump draws washing water from the washing water tank and sprays it onto the dishes via a spray system assigned to the washing zone, in order to remove the dirt adhering to the dishes. The washing water, together with the washed-off dirt, then flows back into the washing water tank. The washed-off dirt is filtered out of the washing water via a sieve system.
In the final-rinse zone, residues of detergent and of dirt located loosely on the dishes are washed off by means of hot fresh water which is sprayed via a corresponding spray system. In some design variants of multi-chamber dishwashers, the after-rinse water is collected in a pump-operated final-rinse tank after the first use and is once more sprayed by means of a pump and by means of a further spray system over the items that are to be cleaned. Chronologically, this process step takes place before the final rinse of the dishes with fresh water. The fresh water or final-rinse water is then returned in part to the washing water tank, in order to dilute the fragments of dirt located there in the washing water tank. The items to be cleaned are then transported into the optional downstream drying zone, in which the items to be cleaned are dried.
Process factors that have a critical influence on the cleaning result are the detergent concentration, the contact time of the items to be cleaned from the first contact with the washing water of the first washing zone until leaving the final-rinse zone, the mechanics of the spray systems and of the spray jets in the washing zones, and the temperatures in the individual washing zones. Methods are known for detecting the process parameters of detergent concentration, contact of the items to be cleaned with the washing water of the first washing zone until leaving the final-rinse zone, and the mechanics in the washing zone. Thus, the detergent concentration is usually detected via the conductance of the washing liquid. The contact time is obtained from the transport speed of the conveyor device, and the washing mechanics are determined via the pressure of the circulation pump and the design of the nozzles of the spray system in the respective washing zone. The temperature of the washing water in the individual treatment zones is detected by temperature sensors. As a result of the washing water cooling after it has left the spray nozzles of the spray system, the temperature that is reached on the surface of the items to be cleaned is not identical to the temperature of the washing water. However, for reducing microorganisms on the surface of the items to be cleaned, it is this temperature reached on the surface of the items to be cleaned in the individual treatment zones that is of critical influence, as also is the time for which these temperatures act on the items to be cleaned. The action of a certain temperature on the surface of the items to be cleaned over a certain period of time is or can be designated as heat equivalent.
The influence of temperature and time on microorganism reduction is one of the bases of regulations and standards that are intended to guarantee the cleaning efficiency of dishwashers. Based on trials carried out on multi-tank dishwashers, with the aim of establishing the process parameters at which reliable hygiene of the items to be cleaned is achieved, Germany adopted standard DIN 10510 C.3, which provides recommendations in respect of temperature, detergent concentration and duration between the first contact of the items to be cleaned with the washing liquid of the first washing zone until leaving the final-rinse zone, with which this multi-chamber dishwasher is to be operated in the individual treatment zones in order for customers to achieve the required microorganism reduction in operation. The basis of this standard is the microorganism reduction of specifically contaminated test specimens after the cleaning process by so-called swab tests. The test microorganism used in this test is E. faecium ATCC 6057.
The testing of hygiene safety of multi-tank dishwashers at the end-user is undertaken by means of swab tests and by determining the microorganism count in the washing water of the last wash tank. However, a disadvantage is the fact that the test of microorganism reduction according to this standard can be carried out at the customer's premises only at considerable expense. A further disadvantage of this standard is the fact that the same microorganism reduction could also be achieved, for example, with a shorter contact time, but at higher temperatures in the individual treatment zones. However, this standard does not permit this.
In the USA, the influence of temperature and time on microorganism reduction is described by the NSF3 standard method. The basis for this standard is the reduction of tuberculosis bacteria determined in trials involving the action of a temperature over time. The action of the temperature over time is designated as “heat equivalent”. How many heat equivalents per second are reached at what temperature is set down in a table in this method. This tables defines a minimum temperature for the washing water of the washing zone and for the after-rinse zone, and dishwashers have to reach these minimum temperatures in order to achieve the microorganism reduction required by this standard. For dishwasher manufacturers, this means that these temperatures have to be preset in the control system of the relevant dishwasher at the time of manufacture and that these temperatures also have to be reached during operation of the multi-tank dishwasher by the customer. When testing a dishwasher according to his method, a temperature sensor is arranged on a plate. The plate is then placed in a predefined position in the transport device of the multi-tank dishwasher and is transported through the individual treatment zones of the multi-tank dishwasher. The temperatures are recorded during the cleaning process. From the temperature profile during the transport of the dishes through the multi-tank dishwasher, and from the abovementioned table, it is possible to determine the heat equivalents acting on the plate throughout the entire cleaning process. This test has to be carried out for three different plate positions in a dish-holding basket or a conveyor belt. To achieve the required microorganism reduction, this standard stipulates that at least 3600 heat equivalents have to be reached in each plate position. An advantage of this method is that the method can be carried out at relatively little expense at the customer's premises for checking that the multi-tank dishwasher is performing correctly in terms of the thermal hygiene. A further advantage is that the result is available immediately after the measurement, and a statement can therefore be made concerning the quality of the cleaning process.
However, a disadvantage in the operation of the dishwasher is the fact that, from the temperatures of the washing water of the individual treatment zones, conclusions have to be drawn concerning the heat equivalents acting on the dishes in the washing process, and not on the heat equivalents actually applied to the items to be washed.
In the field of cleaning and disinfecting appliances, prEN ISO 15883-1 describes a method which, in order to assess hygiene efficiency, also draws on the connection between the microorganism reduction and the temperature over time. This connection is designated as the A0 value and is likewise set down in table form or calculated from a mathematical formula. The A0 value is described in more detail in Annex A of this standard and is defined as the time equivalent in seconds at 80° C. at which a given disinfecting action is exerted, and corresponds analogously to the heat equivalents of the NSF3 standard, but on the basis of another test microorganism. The test microorganism used in this method is Enterococcus faecium. Here too, a minimum A0 value to be reached at any given location in the washing chamber of the cleaning/disinfecting appliance is stipulated. However, this method has not as yet been used for assessing commercial dishwashers in Europe.
The above-described methods and standards for guaranteeing the cleaning result in terms of the thermal hygiene in a multi-tank dishwasher all have the disadvantage that the process parameters are fixed in the operation of the dishwasher. This applies especially to the temperatures in the washing zone and final-rinse zone. If several programs or transport speeds can be selected, the multi-tank dishwasher has to be designed for the worst case. This means in general for the fastest transport speed. The fact that the operation of the multi-tank dishwasher is not based on a fixed method incorporated in the multi-tank dishwasher for the heat equivalents actually acting on the items to be cleaned, which method is connected to the control system of the multi-tank dishwasher and controls the washing process, results in the disadvantage that the multi tank dishwasher in terms of the heat equivalents cannot be optimally adapted to the actual washing process or washing program. A further disadvantage of the presently available prior art is that the heat equivalents actually transmitted to the items to be washed are not detected, and instead it is assumed that the required microorganism reduction is achieved at the process parameters stipulated according to the standard or method.
EP 1 196 650 B1 relates to a method for monitoring a washing process. An independent cableless monitoring device is mounted on a conveyor belt of an industrial dishwasher and is moved along with the belt. The measured data are recorded in a monitoring unit. With this device, the temperatures were able to be recorded at the individual washing zones and evaluated at a later time. The heat equivalents transmitted to the items to be cleaned could then be determined on the basis of the determined temperature values. The difference from the temperature recording according to the NSF3 standard method is that the temperature recording is wireless. However, like the test method according to the NSF3 standard, the device known from EP 1 196 650 B1 serves only to control the process temperatures and is not evaluated by the control system of the dishwasher and thus is not used for direct control of the process parameters of the multi-tank dishwasher.
DE 196 08 036 C5 describes how the amount of after-rinse water is directly dependent on the transport speed of the conveyor device of the multi-tank dishwasher. A dependency of the amount of final-rinse water and the heat equivalents actually transmitted to the items to be cleaned is not dealt with in detail according to DE 196 08 036 C5.